Support n < 2 in `unique.multiPhylo()`

[ms609]

Fixes incorrect behaviour in the case where length(x.multiPhylo) < 1

tree <- read.tree(text = '(a, b);')
c(tree, tree) # = tree
unique(c(tree) # = tree
unique(c(tree)[-1]) # = length zero multiPhylo object

[emmanuelparadis]

Thanks!